fix Passing an unpacked tuple of types as type arguments to the type parameters [T, *Ts] gets error #2363

[asukaminato0721]

Summary

Fixes #2363

Expand unpacked tuple type arguments before specialization so Cls[*tuple[...]] and Cls[Unpack[tuple[...]]] can fill leading type parameters.

Test Plan

Added a regression test covering concrete tuple unpacking into [T, *Ts]

[Copilot]

Pull request overview

Fixes #2363 by allowing unpacked concrete tuple type arguments (via *tuple[...] or Unpack[tuple[...]]) to expand into multiple type arguments before class/type specialization, so they can correctly bind leading type parameters like [T, *Ts].

Changes:

• Expand unpacked tuple type arguments in create_targs before matching them to TParams.
• Add a regression test covering concrete tuple unpacking into [T, *Ts].

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pyrefly/lib/alt/class/targs.rs	Expands unpacked tuple type arguments prior to specialization to allow them to fill multiple parameters.
pyrefly/lib/test/type_var_tuple.rs	Adds a regression test ensuring Cls[*tuple[...]] and Cls[Unpack[tuple[...]]] work with [T, *Ts].

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[yangdanny97]

i think we should have a test case for this

[yangdanny97]

also, it might be good to fix the conformance bug while we're at it

[yangdanny97]

how should we handle Unpack[*tuple[T, ...]]?

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[migeed-z]

@yangdanny97 @asukaminato0721 Is this PR supposed to address this test?

from typing import TypeVar, TypeVarTuple, assert_type

T1 = TypeVar("T1")
Ts = TypeVarTuple("Ts")

TA9 = tuple[*Ts, T1]
TA10 = TA9[*tuple[int, ...]]  # E: Unpacked argument cannot be used for type parameter T1

def func11(a: TA10, b: TA9[*tuple[int, ...], str]):
    assert_type(a, tuple[*tuple[int, ...], int])  # E: assert_type(tuple[Any], tuple[*tuple[int, ...], int]) failed
    assert_type(b, tuple[*tuple[int, ...], str])